High Voltage Direct Current (HVDC) transmission of power has proved to be an efficient alternative to Alternating Current (AC) transmission in many power transmission situations. Typically, in an HVDC power transmission system, an AC/DC converter operating as a rectifier connects an AC power source to one end of an HVDC transmission line, and an AC/DC converter operating as an inverter connects the another end of the HVDC transmission line to an AC grid. Several HVDC transmission lines may be interconnected to form an HVDC transmission network. The higher the voltage used on an HVDC transmission line, the lower will the transmission losses be. Hence, a high voltage transmission is often desirable, in particular when power is transmitted over longer distances. A high voltage across the HVDC transmission line will allow for low-loss transmission of high power.
An AC/DC converter for connecting an AC grid or power source of two or more phases to a DC transmission line typically has one phase leg per AC phase, where a phase leg is arranged to synthesize a sinusoidal voltage waveform from a DC voltage. For high power applications, the phase legs of an AC/DC converter are typically connected in parallel on the DC side.
However, in some applications, it might be desirable to connect an AC grid or power source of lower power to an HVDC transmission line. The voltage withstanding requirements on an AC/DC converter interconnecting an AC system of lower power to an HVDC transmission line will be the same as the voltage requirements on an AC/DC converter interconnecting an AC system of higher power to the same HVDC transmission line, whereas the current with-standing requirements will be lower.
In U.S. Pat. No. 6,519,169, an AC/DC converter having a series connection of phase legs is disclosed. By connecting the phase legs in series, the voltage that each phase leg has to withstand for a particular DC voltage will be reduced compared to an AC/DC converter wherein the phase legs are connected in parallel. For an AC/DC converter having a number P of AC phases, the voltage withstanding requirement on each phase leg will be UDC/P, where UDC is the DC voltage at the DC connection of the AC/DC converter. Thus, the component cost of the AC/DC converter will typically be reduced by connecting the phases in series.
In US 2008/0205093, an AC/DC converter having a series connection of phase legs is proposed, where each phase leg has two parallel cascades of series connected converter cells.